1. Field of the Invention
The present invention relates to a recording apparatus and a recording system that uses an overlapping head in which a plurality of recording heads is arranged to overlap with one another. In particular, the present invention relates to a recording apparatus and a recording system that performs color correction by performing colorimetric measurement of a test pattern.
2. Description of the Related Art
Image recording apparatuses such as a printer use various recording methods. In particular, a printer using an inkjet recording method can be widely applied to consumer products and to large size recording printers for industrial use. In general, an inkjet recording apparatus discharges ink from a recording head including a discharge port array formed of a plurality of discharge ports, and records an image on a recording medium.
Along with the wide spread use of such inkjet recording apparatuses, there are proposals on improving the recording method to improve quality of images recorded by the ink jet recording apparatus. For example, Japanese Patent Application Laid-Open No. 2008-209436 discusses a color correction method in which the test pattern formed of a plurality of patches of different colors is recorded. The recorded patches are read by a density meter or a colorimeter, and correction data is generated based on the read data.
The inkjet printer employs various recording methods, such as a serial recording method for recording by scanning the recording head with respect to the recording medium. Further, there is a full line recording method for recording by conveying the recording medium with respect to a full line head formed by arranging a plurality of recording head chips. A width that can be recorded by the full line recording head corresponds to the width of the recording medium in the full line recording method. Recording can thus be performed by conveying the recording medium in a direction perpendicular to the direction of the discharge port array, so that high speed recording can be performed.
A recordable area of such a full line recording head can be elongated by connecting recording head chips of the same color. According to the present invention, a full line head formed by connecting a plurality of recording head chips will be referred to as an overlapping head. In general, the overlapping head is formed by overlapping and connecting a portion (i.e., an overlapping portion) of each of the plurality of recording head chips. The overlapping head then records by distributing the image data to be recorded to the overlapping portions of each of the recording head chips.
Conventionally, there are various methods for reducing color unevenness or streaks caused by an assembly error of the recording head chips that occurs when the overlapping head is manufactured. Such color unevenness and streaks will be described in detail below with reference to FIGS. 2A, 2B, and 2C.
FIG. 2A illustrates an overlapping head formed by connecting recording head chips A and B. According to the present invention, overlapping portions of each of the chips which overlap with each other will be referred to as overlapping portions, and the portions that do not overlap will be referred to as non-overlapping portions. Referring to FIG. 2A, there are overlapping portions 172 and 173, and non-overlapping portions 171 and 174. It is necessary to assemble the overlapping head so that the positions of the overlapping portions 172 and 173 completely overlap. However, the overlapping portions of each of the chips may become slightly displaced due to the assembly error that occurs when manufacturing the overlapping head.
If the image data is then distributed to such an overlapping head including displacement, image quality is degraded. FIG. 2B illustrates an example in which the image data to be recorded by the overlapping portions of the chips A and B are distributed so that distribution rate of the discharge ports in the overlapping portion 172 to those in the overlapping portion 173 becomes 50%-50%. FIG. 2B indicates that, when there is displacement due to the assembly error, there is an area which is recorded by only one of the chips, i.e., only 50% of the image data is recorded in the area.
As a result, a white streak as illustrated in FIG. 1 is generated in a boundary region between the area recorded by the overlapping portion and the area recorded by the non-overlapping portion of the overlapping head. The white streak is generated in the example illustrated in FIG. 2B due to the displacement caused by the overlapping portions of the two chips that are apart from each other. However, if the displacement is generated by the overlapping portions excessively overlapping with each other, the image data becomes recorded by 150%, so that a black streak is generated.
To solve such image degradation, there is a method for distributing the image data to be recorded by the overlapping portions so that the distribution rate between each of the discharge ports gradually changes. This is as illustrated in FIG. 2C. For example, Japanese Patent Application Laid-Open No. 2007-152582 discusses such a method. A drastic increase in the number of discharge ports to be used is reduced by gradually distributing the image data. The generation of the streak in the boundary portion between the overlapping portion and the non-overlapping portion is thus reduced.
On the other hand, the inventors of the present invention have found that a problem other than the white streak and the black streak is generated as image degradation due to the assembly error of the recording head chip. More specifically, the colors (i.e., elements such as a color hue, saturation, and intensity) become different by the displacement of impact positions of ink droplets due to the assembly error. In such a case, the color recorded by the overlapping portions becomes different from the color recorded by the non-overlapping portions, even when the color of the image recorded by the non-overlapping portions of the chips is the same. Such a case will be described in detail below with reference to FIGS. 3A, 3B, 3C, and 3D.
FIG. 3A illustrates a state in which there is a small assembly error in the recording head chips A and B. Referring to FIG. 3A, the recording head chips A and B discharge the same amount of the same color ink, and the color to be recorded by the each of the non-overlapping portions is the same. FIGS. 3B and 3C illustrate the image recorded by the overlapping portions, i.e., an arbitrary pixel whose gradation is expressed by nine dots. Such an image is created by distributing the image data to the chip A and the chip B using a staggered pattern mask, and all of the nine dots are printed in the image.
More specifically, FIG. 3B illustrates an example of a case where there is no assembly error, as compared to FIG. 3C in which there is an assembly error. Referring to FIG. 3B, the impact error of the ink dots due to the assembly error is not generated, and the dots are printed without overlapping with each other.
On the other hand, referring to FIG. 3C, since the impact positions of the ink dots are displaced due to the assembly error, the dots discharged from the chip A and the dots discharged from the chip B overlap on the recording medium. Since the color of the image becomes different depending on coverage or an overlapping rate of the dots on the recording medium, the colors of the pixels become different even when both are formed of the same nine dots.
In other words, when there is no assembly error as illustrated in FIG. 3B, the colors generated by the non-overlapping portions and the overlapping portions become the same. However, if there is an assembly error as illustrated in FIG. 3C, the colors generated by the non-overlapping portions and the overlapping portions become different. Further, if a plurality of such pixels is collected together, a considerable difference between the colors becomes generated.
Such a difference between the colors of the non-overlapping portions and the overlapping portions may be generated for all overlapping portions. More specifically, when the recording head formed by connecting a plurality of the same color chips is used in recording as illustrated in FIG. 3D, a color deviation is generated due to the impact error. The color deviation is generated at a number of positions equal to the number of overlapping portions (i.e., number of chips—1). The color deviation becomes visible as a streak of a different color on the recorded image and thus causes image degradation. It thus becomes necessary to assemble the recording head chips with high precision to prevent generation of the color deviation, which increases cost.
Such a color deviation caused by the overlapping dots cannot be reduced even when using a gradation mask discussed in Japanese Patent Application Laid-Open No. 2007-152582. As described above, the gradation mask is used to gradually distribute the image data to be recorded to the discharge ports of the overlapping portions so that discharge port distribution rate gradually changes. The gradation mask thus does not reduce the overlapping caused by the displacement in the dot impact positions.
In general, the color deviation can be corrected using a correction method discussed in Japanese Patent Application Laid-Open No. 2008-209436. More specifically, colorimetric measurement is performed on the test patterns recorded by each of the overlapping portions and the non-overlapping portions, and the difference in the colors is reduced by performing color correction based on the colorimetric results.
However, when the discharge port distribution rate of the overlapping portions is determined using the gradation mask discussed in Japanese Patent Application Laid-Open No. 2007-152582, the difference in the colors cannot be accurately corrected using the correction method discussed in Japanese Patent Application Laid-Open No. 2008-209436. Such a case will be described in detail below with reference to FIGS. 4A, 4B, 4C, 4D, and 4E.
FIG. 4A illustrates the overlapping head formed by connecting the recording chip A and the recording chip B. The image data to be recorded by the overlapping portions is then distributed using the gradation mask that gradually distributes the image data as illustrated in FIG. 4B. FIGS. 4C, 4D, and 4E illustrate a plurality of patches recorded using such an overlapping head. FIG. 4C illustrates the patch recorded by the non-overlapping portion of the chip A, and FIG. 4E illustrates the patch recorded by the non-overlapping portion of the chip B. Each of such patches is recorded by only one of the chips. FIG. 4D illustrates the patch recorded by the gradation mask distributing the image data to the overlapping portions of the chip A and the chip B.
Referring to FIGS. 4C and 4E, the patches recorded by the non-overlapping portions are of the same color, and the color within each patch is uniform. On the other hand, referring to FIG. 4D, the color of the image recorded in the patch by the overlapping portions is different from the patch recorded by the non-overlapping portion due to the overlapping of the dots caused by the impact position displacement.
In particular, since the gradation mask distributes the image data, the discharge port distribution rate of the two chips gradually changes in the direction of the discharge port array. As a result, the displacement of the impact position, i.e., an amount of the dots overlap, becomes different depending on the position, so that the color of the image within the patch does not become uniform. In other words, if the gradation mask is used for the overlapping portions, the color unevenness is generated within the recorded patch. As a result, the color deviation cannot be accurately corrected even when using the colorimetric data acquired by performing colorimetric measurement of the patch.
Further, the overlapping portion is a very narrow region, i.e., 1 to 2% of the recording head chip. For example, if the width of the recording head chip is 1 inch, the width of the overlapping portion is approximately 2 mm. As a result, a current colorimeter cannot correctly perform colorimetric measurement of an image of a narrower width, even when the patch is recorded by dividing the area of an uneven color into a plurality of areas.